This invention relates to tube repair methods and more particularly to methods for preparing and attaching a sleeve within a tube by a brazing procedure.
In tube-type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak occurs therein which allows the fluids to mingle. When this occurs, it is sometimes necessary to plug the tube so that the fluid does not flow through the tube, thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube-type heat exchangers are commonly referred to as steam generators. When a defect occurs in the tubes of a nuclear steam generator that allows the coolant in the tubes to mingle with the coolant outside of the tubes, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of a nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube must either be plugged or repaired so that the coolant does not leak from the tube. This prevents contamination of the fluid surrounding the tubes.
There are several methods known in the art for repairing heat exchange tubes, however, many of these methods are not applicable to repair of heat exchange tubes wherein the tube is not readily accessible. For example, in a nuclear steam generator the physical inaccessibility of defective heat exchange tubes and the radioactive nature of the environment surrounding the heat exchange tubes presents unique problems to repairing heat exchange tubes that do not normally exist in other heat exchangers. For these reasons, special methods have been developed for repairing heat exchange tubes in nuclear steam generators. Typically, the method used to repair a heat exchange tube in a nuclear steam generator is one in which a metal sleeve having an outside diameter slightly smaller than the inside diameter of the defective tube is inserted into the defective tube and attached to the defective tube to bridge the defective area of the tube. This type of repair method is generally referred to as "sleeving". Previous sleeving development work has been concerned with obtaining a relatively leakproof joint between the sleeve and the tube by brazing, arc welding, explosive welding, or other joining means. Due to the need for cleanliness, close fittings, heat application, and atmospheric control, these metallurgical bonding techniques have problems which are not easily solvable in areas such as nuclear steam generators where human access is limited. In addition, there are many methods known in the art for preparing the sleeves prior to insertion and brazing; however, those methods do not address the problem of preparing a sleeve for use in a nuclear environment.
While there are methods known in the art for preparing tubes for brazing, for inserting sleeves within tubes of heat exchangers, and for brazing tubes together, there do not exist methods for preparing, inserting and brazing sleeves in nuclear steam generator heat exchange tubes which are capable of being performed where personnel access is limited and where a leaktight joint is needed. Therefore, what is needed is a method for preparing and securing a sleeve within a heat exchange tube of a nuclear steam generator by brazing that produces a leaktight joint between the sleeve and the tube so that no leakage occurs from the tube in the defective region.